1. Field of the Invention
The present invention relates generally to in-circuit impedance measurement. More particularly, the present invention relates to a method and apparatus for measuring an unknown impedance without removing the components from a circuit.
2. Related Art
It is often desirable to test a discrete component which is part of a larger circuit. The measurement of a component""s impedance is useful in determining whether or not a component is functioning properly, and may be used in determining other operating characteristics of the component. In addition, such measurement may be utilized to verify whether or not a correct component was loaded on a printed circuit board.
The testing of circuits and components may be classified generally into two categories, namely analog and digital. Analog testing refers to the measurement of quantitative and continuous signals. For example, the measurement of voltage, current, and frequency are typically classified as analog measurements. Analog testing is sometimes referred to as dynamic testing when time or another dynamic variable parameter is involved. In addition, analog testing is sometimes referred to as static testing when a fixed, non-dynamic variable parameter is involved.
The process of functional testing includes applying a signal similar to the signal which would normally be experienced in the intended application to the input terminals of a device under test. The output signal is then determined to be acceptable or not for normal operation. In digital functional testing, a digital output is examined to determine if it matches the expected pattern of 1s and 0s. In analog functional testing, the output signals are measured to determine if their incremental levels fall within the acceptable limits of time, voltage, current or other parameters.
Separate components may be easily tested or measured by themselves. For example, measurement of impedance may be accomplished by connecting the component directly to a meter. A known current may be applied through the component and the voltage across the component measured. The impedance is the ratio of voltage to current.
Special problems arise when testing a component in-circuit, because of the interaction of the component with the remainder of the circuit to which it is connected. For example, a current applied between two nodes or across a component flows only partially through the component, because the current will be shared with all the elements in the circuit. Thus, the impedance for the individual element cannot be directly determined by the voltage across the element because the current through the element is unknown.
Various devices and methods have been developed in an attempt to test or measure components in-circuit. Such devices and methods tend to be complicated, cumbersome, and expensive. Such testing may be accomplished as a verification of design. For example, a voltage or current may be applied at two or more nodes, and the voltages of all other nodes are measured and compared to a design simulation. If the nodes are within a tolerance window the components of the circuit are presumed valid.
Presently, there is no known effective method or device for providing the accurate measure of a single unknown component while in a circuit. If the component""s value is to be determined accurately, it is removed from the circuit and tested as a separate element using a classical impedance measuring instrument, such as a resistance-inductance-capacitance (RLC) meter or an Ohm meter.
It has been recognized that it would be advantageous to develop a method and apparatus for accurately measuring an unknown impedance in-circuit, without removing components from the circuit.
The invention provides an in-circuit impedance measurement method and device to measure an unkown impedance in an electrical circuit between first and second nodes. The method and device advantageously accurately measures the impedance in-circuit, without removing components from the circuit. The method and apparatus of the present invention advantageously isolate a single impedance between the two nodes of the circuit from the remaining circuitry by probing the two nodes, and probing at a common node where the current is the same as the current between the first two nodes.
The device includes at least one current source which provides respective first and second currents or current signals of known values. Preferably, first and second current sources may be provided. The currents may be different or applied at different times. Preferably, the currents are good quality sine waves.
First and second probes are connected to the current source or respective first and second current sources. The first and second probes contact the respective first and second nodes to apply the first and second currents to the first and second nodes. A third common probe is connected to the current source, or first and second current sources, to contact the circuit at a common node at a location that has an equal current flow as between the first and second nodes.
At least one voltage meter is connected to the first and second nodes to measure voltages corresponding to the first and second currents. Preferably, first and second voltage meters are provided. The first voltage meter measures two separate voltages at the first node due to the first current source and the second current source. Similarly, the second voltage meter measures two separate voltages at the second node due to the first current source and the second current source.
In accordance with one aspect of the present invention, the first and second voltage meters may be connected to the respective first and second probes, and to the third common probe. Alternatively, fourth and fifth probes may connect the first and second voltage meters to the first and second nodes, while a sixth common probe connects the first and second voltage meters to the circuit at the common node at the location that has an equal current flow as between the first and second nodes.
In accordance with another aspect of the present invention, a signal processor may be coupled to the voltage meters to isolate the voltages measured by the voltage meters caused by the first and second currents. In addition, the signal processor may calculate the impedance of the component based on the known values of the first and second currents and the measured voltages.
In accordance with another aspect of the present invention, the first and fourth probes may be disposed on a first single probe body, but define separate electrical paths with separate electrical contact points. Similarly, the second and fifth probes may be disposed on a second single probe body, but define separate electrical paths with separate electrical contact points. In addition, the third and sixth probes may be disposed on a third single probe body, but define separate electrical paths with separate electrical contact points.
In accordance with another aspect of the present invention, the device may be self-calibrating. A reference resistor may be selectively connected to the current sources and voltage meters by at least one switch.
In accordance with another aspect of the present invention, an internal shunt may be provided for situation or circuits in which no pi network is available. The current sources may include a plurality of unbalanced resistors to create the internal shunt.
In accordance with another aspect of the present invention, the current may be measured or determined to verify proper contact between the probes and the circuit.
A method for measuring the impedance of the discrete component includes selecting a common node at a location that experiences an equal current flow with respect to the component or the impedance between the two nodes. In addition, the method includes applying the first and second currents or current signals of known values at the respective first and second nodes. The current may be applied to the circuit by contacting the first and second nodes with the respective first and second probes, and contacting a common node with the third common probe. The currents applied may be different first and second current signals. In addition, the first and second currents may be applied at different times.
Four voltages are measured, including two voltages at each of the first and second nodes, corresponding to the first and second current sources. Measuring the four voltages may include measuring (1) the voltage between the first and common nodes caused by the first current; (2) the voltage between the first and common nodes caused by the second current; (3) the voltage between the second and common nodes caused by the first current; and (4) the voltage between the second and common nodes caused by the second current. The voltages may be measured by contacting the first and second nodes with the respective fourth and fifth probes, and contacting a common node with the sixth common probe.
The impedance of the component is calculated based on the known values of the first and second currents, and the measured values of the four voltages. The component is treated as a cross member of a pi network. The remaining circuitry and measurement probes are treated as side shunts of the pi network containing the parallel effects of all components in the circuit and the loading of the probes. The voltages corresponding to the first and second currents may be separated or isolated from each other and from any noise.
In accordance with another aspect of the method of the present invention, the impedance of the component may be measured while the circuit is active. The circuit is first activated and system signals present in the circuit are analyzed. Frequencies for the currents are selected which will not conflict with the system signals, or which will have predictable conflicts with the system signals prior. The voltages caused by the currents may then be separated or isolated.
In accordance with another aspect of the method of the present invention, the device may be calibrated by switching the current sources and voltage meters to a reference resistor.
In accordance with another aspect of the method of the present invention, the contacts between the probes and the circuit may be verified by measuring or determining the current. In accordance with another aspect of the method of the present invention, an internal shunt may be provided by unbalancing a plurality of resistors forming the current sources.
Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention.